/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/MeshTangents.h
Per-Triangle-Vertex Tangents Number of Tangents is NumTriangles * 3 Set internal buffer sizes suitable for calculating per-triangle tangents. This is intended to be used if you wish to calculate your own tangents and useSetPerTriangleTangent()
void InitializeTriVertexTangents ( bool bClearToZero )